Motor and turbo-molecular pump

ABSTRACT

A compact turbo-molecular pump having a high depressurizing capability. A motor for rotating a rotor vane includes an air bearing. The air bearing has a rotary cylinder and a fixed surface surrounding the rotary cylinder. The material of the rotary cylinder has a coefficient of thermal expansion that is smaller than that of the material of the fixed surface. Thus, change in the dimensions of the rotary cylinder is smaller than that of the fixed surface even if the temperature of the air bearing rises during operation of the pump. Thus, the rotary cylinder avoids contact with the fixed surface.

This is a divisional application of application Ser. No. 09/493,983,filed Jan. 28, 2000 now U.S. Pat. No. 6,664,683.

BACKGROUND OF THE INVENTION

The present invention relates to a motor and a turbo-molecular pump.

A turbo-molecular pump produces an ultra-high vacuum state and isemployed in, for example, semiconductor fabrication related apparatuses(e.g., sputtering apparatuses, chemical vapor deposition (CVD)apparatuses, and etching apparatuses) and measuring apparatuses (e.g.,electron microscopes, surface analysis apparatuses, and environmenttesting apparatuses). A typical turbo-molecular pump includes a motorand a plurality of rotor vanes rotated by the motor. The rotor vanes arerotated to produce a molecular flow and discharge gases. This causes anultra-high vacuum state in the interior of the apparatus connected tothe turbo-molecular pump.

The motor has a rotary shaft that is rotated at a high speed to producethe ultra-high vacuum state. The bearing that supports the rotary shaftmust thus be capable of high speed rotation. A ball bearing, whichrequires lubricating oil, is not appropriate for such application. Thisis because the vapor pressure of the lubricating oil, although low,hinders depressurization to the ultra-vacuum state by theturbo-molecular pump. Further, vaporized lubricating oil contaminatesvacuum chambers.

Japanese Unexamined Utility Model Publication No. 63-14894 and JapaneseUnexamined Patent Publication No. 2-16389 describes a turbo-molecularpump that does not use lubricating oil. This turbo-molecular pumpemploys non-contact bearings, such as air bearings or magnetic bearings.

A kinetic air bearing is one example of an air bearing. This air bearinghas a fixed cylinder and a rotatable cylinder, which is arranged in thefixed cylinder. A bearing area and a seal area are defined on the outersurface of the rotatable cylinder. A plurality of dynamic pressuregrooves extend along the bearing area. A predetermined clearance isprovided between the outer surface of the rotatable cylinder and theinner surface of the fixed cylinder.

One end of the two cylinders is exposed to a predetermined vacuumatmosphere. Thus, the seal area is located near that end of therotatable cylinder to prevent gases from moving between a compressed gaslayer in the bearing and the vacuum atmosphere. A plurality of helicalseal grooves extend along the seal area. An annular groove formed on theouter surface of the rotatable cylinder extends along a boundary areadefined between the bearing area and the seal area. An aperture extendsthrough the fixed cylinder at a location opposed to the annular groove.The motor drives and rotates the rotatable cylinder. The rotation causesthe air outside the fixed cylinder to pass through the aperture and intothe clearance (especially to the region between the bearing area and theopposed area of the fixed cylinder). This forms a pressure gas film, orthe compressed gas layer, which radially supports the rotary shaft.

When the rotating speed of the rotatable cylinder is lower than apredetermined value, the rotatable cylinder slides on the fixedcylinder. Ceramics having relatively high anti-wear properties, such asalumina and zirconia, may be used as the material of the fixed androtatable cylinders.

When designing the turbo-molecular pump, the depressurizing capabilityof the motor determines the number of vanes and the motor speed. Forexample, the motor speed is 50,000 rpm to 70,000 rpm in a typical,compact turbo-molecular pump.

The viscous friction produced by air increases the temperature of theair bearing during high speed rotation. The generated heat istransferred rather easily from the outer surface of the fixed cylinder.On the other hand, since the rotatable cylinder is covered by the fixedcylinder, heat cannot be transferred from the rotatable/cylinder soeasily. This results in a large difference between the temperature ofthe fixed cylinder and the temperature of the rotatable cylinder. Thecoefficient of thermal expansion for ceramics, such as alumina andzirconia, is 7 to 8×10⁻⁶/° C. and thus relatively high. Therefore, in anair bearing made of alumina or zirconia, the temperature differencebetween the fixed cylinder and the rotatable cylinder causes thedimension change of the fixed cylinder to differ from that of therotatable cylinder. This varies the size of the clearance. Consequently,the rotatable cylinder may contact the fixed cylinder and obstruct highspeed rotation.

A fan is often used to cool the air bearing. The fan is effective forcooling the outer part of the air bearing, or the fixed cylinder, butnot for cooling the inner part, or the rotatable cylinder. Hence, thefan further increases the temperature difference between the rotatablecylinder and the fixed cylinder and changes the size of the clearance.There is a demand for a more compact turbo-molecular pump that operatesat higher rotating speeds. In such a pump, the size of the clearancemust be decreased. Therefore, the effects of heat on the air bearingcannot be ignored.

To increase the speed of the motor, a bearing that has an improved sealand improved performance is necessary. The vibrations of the rotatablecylinder affect the supporting characteristics of the rotatablecylinder. For example, the depth of the dynamic pressure grooves affectthe natural frequency of the rotatable cylinder. When the naturalfrequency (Hz) of the rotatable cylinder and the rotating speed (rps) ofthe rotatable cylinder are about the same, the possibility of resonanceis high. Resonance causes vibrations of the motor. Therefore, the depthof the dynamic pressure grooves is an important factor for obtainingimproved bearing characteristics. Further, the depth of the seal groovesaffects the seal characteristics. Hence, the depth of the seal groovesis an important factor for obtaining a high degree of vacuum.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compact motorapplicable to high speeds and a compact turbo-molecular pump having ahigh depressurizing capability.

To achieve the above object, the present invention provides a motorincluding a rotary shaft and a bearing for radially supporting therotary shaft. The bearing includes a cylindrical rotary member connectedto the rotary shaft, and a cylindrical fixed surface surrounding therotary member. The fixed surface is spaced from the rotary member by apredetermined distance. The material of the rotary member has acoefficient of thermal expansion that is smaller than that of thematerial of the fixed surface.

Another aspect of the present invention provides a motor including arotary shaft and a bearing for radially supporting the rotary shaft,wherein the bearing includes a cylindrical rotary member connected tothe rotary shaft and a cylindrical fixed surface surrounding the rotarymember. The fixed surface is spaced from the rotary member by apredetermined distance. The rotary member is made of a material having acoefficient of thermal expansion that is 5×10⁻⁶/° C. or less.

A further aspect of the present invention provides a turbo-molecularpump including a housing, a stator vane attached to the housing, a rotorvane rotated relative to the stator vane, and a motor for driving therotor vane. The motor includes a rotary shaft and a bearing for radiallysupporting the rotary shaft. The bearing includes a cylindrical rotarymember connected to the rotary shaft and a cylindrical fixed surfacesurrounding the rotary member. The fixed surface is spaced from therotary member by a predetermined distance. The material of the rotarymember has a coefficient of thermal expansion that is smaller than thatof the material of the fixed surface.

A further aspect of the present invention provides a turbo-molecularpump including a housing, a stator vane attached to the housing, a rotorvane rotated relative to the stator vane, and a motor for driving therotor vane. The motor includes a rotary shaft and a bearing for radiallysupporting the rotary shaft. The bearing includes a cylindrical rotarymember connected to the rotary shaft and a cylindrical fixed surfacesurrounding the rotary member. The fixed surface is spaced from therotary member by a predetermined distance. The rotary member is made ofa material having a coefficient of thermal expansion that is 5×10⁻⁶/° C.or less.

A further aspect of the present invention provides a motor including arotary shaft and a bearing for radially supporting the rotary shaft. Thebearing includes a cylindrical rotary member connected to the rotaryshaft and a cylindrical fixed surface surrounding the rotary member. Thefixed surface is spaced from the rotary member by a predetermineddistance. At least one of the rotary member and the fixed surface has adynamic pressure groove formed on a predetermined first area defined ona surface opposing the other of the rotary member and the fixed surface.At least one of the rotary member and the fixed surface has a sealgroove formed on a predetermined second area defined on a surfaceopposing the other one of the rotary member and the fixed surface. Theseal groove is formed deeper than the dynamic pressure groove.

A further aspect of the present invention provides a turbo-molecularpump including a housing, a stator vane attached to the housing, a rotorvane rotated relative to the stator vane, and a motor for driving therotor vane. The motor includes a rotary shaft and a bearing for radiallysupporting the rotary shaft. The bearing includes a cylindrical rotarymember connected to the rotary shaft and a cylindrical fixed surfacesurrounding the rotary member. The fixed surface is spaced from therotary member by a predetermined distance. At least one of the rotarymember and the fixed surface has a dynamic pressure groove defined on asurface opposing the other of the rotary member and the fixed surface.At least one of the rotary member and the fixed surface has a first sealgroove formed on a surface opposing the other of the rotary member andthe fixed surface. The first seal groove is formed deeper than thedynamic pressure groove.

Other aspects and advantages of the present invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

FIG. 1 is a cross-sectional view showing a turbo-molecular pumpaccording to a first embodiment of the present invention;

FIG. 2 is a cross-sectional view showing a brushless motor of theturbo-molecular pump of FIG. 1;

FIG. 3 is a side view, partly in cross-section, showing an air bearingof the brushless motor of FIG. 2;

FIG. 4 is a graph showing the relationship between the size of the airbearing clearance and the temperature difference between a rotarycylinder and a fixed surface;

FIG. 5 is a graph showing the relationship between the size of an airbearing clearance and the temperature difference between a rotarycylinder and a fixed surface in a turbo-molecular pump according to asecond embodiment of the present invention;

FIG. 6A is a side view, partly in cross-section, showing an air bearingof a turbo-molecular pump according to a third embodiment of the presentinvention;

FIG. 6B is a diagram showing the depths of the grooves formed in therotary cylinder of FIG. 6A;

FIG. 7 is a graph showing the relationship between the seal groove depthand the groove inlet pressure; and

FIG. 8 is a graph showing the relationship between the dynamic pressuregroove depth and the natural frequency of the rotary cylinder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A turbo-molecular pump 1 according to a first embodiment of the presentinvention will now be described with reference to FIGS. 1 to 4.

As shown in FIG. 1, the turbo-molecular pump 1 has a housing 2 and abrushless motor 10. The housing 2 includes an upper housing 3, a lowerhousing 4, and an annular support block 5. The support block 5 is heldbetween the upper and lower housings 3, 4. The upper housing 3, thesupport block 5, and the lower housing 4 are fastened together by aplurality of bolts 6.

An intake 3 a, which is connected to a vacuum chamber (not shown),extends through the top end of the upper housing 3. Gas in the vacuumchamber is drawn into the motor 10 through the intake 3 a. A pluralityof supports 8 are attached to the inner wall of the upper housing 3. Thesupports 8 support a plurality of stator vanes 7, each of which extendsinwardly from the upper housing 3. An annular support adapter 9 is fixedto the lower surface of the support block 5.

O-rings 11, 12 are fitted in the inner surfaces of the support block 5and the support adapter 9, respectively. The motor 10 is supported bythe support block 5 and the support adapter 9. The O-rings 11, 12 sealthe space between the motor 10, the support block 5, and the supportadapter 9. An intake region 18 is defined above the support block 5. Adischarge region 19 is defined below the support block 5 and the supportadapter 9.

The motor 10 has a rotary shaft 13, the upper end of which extends intothe intake 3 a. A cup-like wheel 14 is fastened to the upper end of therotary shaft 13 by a nut 15. A plurality of rotor vanes 16 extend fromthe peripheral surface of the wheel 14. The rotor vanes 16 are eacharranged in a gap formed between an associated pair of stator vanes 7.The motor 10 includes a motor case 17, which is spaced slightly from theinner cylindrical surface of the wheel 14.

An outer helical groove 17 a extends along the outer surface of themotor case 17 and is opposed to the inner surface of the wheel 14. Aplurality of air cooling slits 17 b extend through part of the outersurface of the motor case 17 in the discharge region 19.

An air cooling fan unit 20 is attached to the lower housing 4. The fanunit 20 has a motor 21 and a fan 22, which is connected to the motor 21.The fan unit 20 produces an air current directed toward the slits 17 b.

The structure of the brushless motor 10 will now be described.

As shown in FIG. 2, a pair of annular plugs 23, 24 are fitted into theends of the motor case 17. An upper bore 23 a extends through the centerof the upper plug 23, and a lower bore 24 a extends through the centerof the lower plug 24. The bores 23 a, 24 a receive the rotary shaft 13.

A rotary element 25, which is fixed to the shaft 13, rotates integrallywith the shaft 13. The rotary element 25 includes a field magnet 26, apair of bushings 27 a, 27 b, a rotary tube, or cylinder 28, and a pairof rotary magnets 39, 40. The field magnet 26 has four permanent magnetplates (not shown). The four permanent magnet plates are parallel to oneanother and extend axially about the shaft 13. Further, the fourpermanent magnet plates are arranged to form a cylindrical shape. Thepolarity of each magnet plate differs from that of the adjacent magnetplates in an alternating manner. The rotary cylinder 28, or rotarymember, covers the field magnet 26. The two bushings 27 a, 27 b seal thespace between the rotary cylinder 28 and the shaft 13 and hold the fieldmagnet 26 in between. The bushings 27 a, 27 b function to adjust therotating balance of the rotary element 25.

A cylindrical fixed tube, or surface 29 is secured to the motor case 17to cover the rotary element 25. The fixed surface 29 and the shaft 13are coaxial. The inner surface of the fixed surface 29 is ground toimprove its friction characteristics. The rotary cylinder 28 and thefixed surface 29 form an air bearing 30 that radially supports therotary shaft 13.

Three equally spaced armature coils 31 are arranged about the peripheralsurface of the fixed surface 29. The angular dimension of each armaturecoil 31 is within a range of 90° to 120°. A cylindrical yoke 32 issecured to the inner surface of the motor case 17 to surround thearmature coils 31.

Three magnetic sensors (Hall devices) 33 are arranged along the outersurface of the fixed surface 29 in correspondence with the threearmature coils 31. The three armature coils 31 and the three magneticsensors 33 are each connected to a plurality of connector pins 34 (onlyone shown in FIG. 2), which extend through the plug 24. The connectorpins 34 are electrically connected to an external control circuit (notshown). The magnetic sensors 33 detect changes in the polarity of thefield magnet 26. The control circuit detects the rotating speed based onthe detection signal of the magnetic sensors 33 and controls the currentprovided to the armature coils 31 accordingly to control the rotatingspeed of the shaft 13.

A pair of magnetic bearings 37, 38 support the rotary shaft 13 in anon-contacting manner. The upper magnetic bearing 37 includes the upperrotary magnet 39 and an upper fixed magnet 41, which is fixed to theupper plug 23. The lower magnetic bearing 38 includes the lower rotarymagnet 40 and a lower fixed magnet 42, which is fixed to the lower plug24. The facing surfaces of the magnets 39, 41 have the same polarity.Thus, the magnets 39, 41 repel one another. Further, the facing surfacesof the magnets 40, 42 have the same polarity and the magnets 40, 42repel one another. The magnets 39-42 are all made of the same material,preferably from a neodymium magnet or a samarium magnet.

The rotary cylinder 28 and the fixed surface 29 are coaxial. A clearancehaving a predetermined radial distance C is provided between the rotarycylinder 28 and the fixed surface 29. The clearance distance C ispredetermined in accordance with the capability of the motor 10 (e.g.,the motor rotating speed). If the rotating speed of the motor 10 is in arange of 60,000 rpm to 90,000 rpm, the clearance distance C is 10 μm orlower. If the rotating speed of the motor 10 is in a range of 50,000 rpmto 70,000 rpm, the clearance distance C is 3 μm to 6 μm. In the firstembodiment, the clearance distance C is set to 5 μm.

The rotary cylinder 28 and the fixed surface 29 are made of sinteredceramic. More specifically, the rotary cylinder 28 is made of siliconcarbide, and the fixed surface 29 is made of alumina. The coefficient ofthermal expansion of silicon carbide is 3 to 4×10⁻⁶/° C. The coefficientof thermal expansion of alumina is 7 to 8×10⁻⁶/° C. In other words, thecoefficient of thermal expansion of the material used for the fixedsurface 29 is greater than that of the material used for the rotarycylinder 28.

As shown in FIG. 3, an upper air bearing area 43 a, a lower air bearingarea 43 b, and a gas seal area 44 are defined on the outer surface ofthe rotary cylinder 28. The gas seal area 44 is located closer to theupper end of the rotary cylinder 28 than the upper air bearing area 43a. A plurality of equally spaced V-shaped grooves (dynamic pressuregrooves) 45 a, 45 b, which are arranged in a herringbone pattern, extendcircumferentially along the upper and lower air bearing areas 43 a, 43b, respectively. A helical groove (seal groove) 46 extends along the gasseal area 44. An annular groove 47 a extends between the upper airbearing area 43 a and the gas seal area 44, and an annular groove 47 bextends between the upper and lower air bearing areas 43 a, 43 b.

Referring to FIGS. 2 and 3, a plurality of apertures 48 extend throughthe fixed surface 29 at locations corresponding to the annular grooves47 a, 48 a. When the motor 10 is driven, the V-shaped grooves 45 a, 45 bfunction to draw the air outside the fixed surface 29 through theapertures 48 and toward the outer surface of the rotary cylinder 28. Asthe rotating speed of the rotary element 25 increases, the amount of airdrawn into the clearance between the rotary cylinder 28 and the fixedsurface 29 increases thereby forming compressed gas layers. Thecompressed gas layers prevent the rotary element 25 from contacting thefixed surface 29 during rotation.

As the motor 10 drives the rotary shaft 13 and rotates the wheel 14, theair in the intake 3 a is drawn into the space between the rotor vanes 16and the stator vanes 7. The air is drawn into the outer helical groove17 a of the motor case 17 and into the space between the rotary shaft 13and the motor case 17. Then, the air flows through the inner helicalgroove 46 and the apertures 48 into a gap 36 formed between the fixedsurface 29 and the motor case 17. The air is then released into thedischarge region 19 through the slits 17 b. The released air isdischarged through vent holes 4 a formed in the lower housing 4. As theair passes through the outer helical groove 17 a and the inner helicalgroove 46, the velocity of the air increases significantly. The velocitychange increases the fluid drawing effect. The outer helical groove 17 aand the inner helical groove 46 extend in a direction corresponding tothe rotation direction. Thus, the flow of air in the grooves 17 a, 46 isrestricted to one direction. This prevents reversed air flow in thegrooves 17 a, 46 and increases the depressurization capability of thepump 1.

When the motor 10 is driven, the V-shaped grooves 45 a, 45 b function toform a high-pressure compressed gas layer at the axially middle portionof each air bearing area 43 a, 43 b. The compressed gas layer causes therotary element 25 to float away from the inner surface of the fixedsurface 29 when the rotating speed approaches 5000 rpm. In this state,the rotary shaft 13 (the rotary element 25) is radially supported by theair bearing 30.

When a certain amount of time elapses after the motor 10 reaches anormal rotating speed, the viscous friction of air heats the rotarycylinder 28 and the fixed surface 29. In this state, the air currentproduced by the fan unit 20 passes through the slits 17 b and toward thefixed surface 29, which cools the fixed surface 29 with forced air.Hence, the temperature increase of the fixed surface 29 is relativelysmall. The rotary cylinder 28 is not cooled with forced air. Hence, thetemperature increase of the rotary cylinder 28 is relatively large.Accordingly, when the pump 1 is operated, the temperature differencebetween the rotary cylinder 28 and the fixed surface 29 reachesapproximately 80° C. to 100° C.

However, the coefficient of thermal expansion of the rotary cylinder 28is smaller than that of the fixed surface 29. Accordingly, the change inthe outer diameter of the rotary cylinder 28 is about the same orslightly greater than that of the inner diameter of the fixed surface29. Thus, the temperature-related change in the distance C between thefixed surface 29 and the rotary cylinder 28 is small.

FIG. 4 is a graph showing the relationship between the clearancedistance C (μm) and the temperature difference ΔT (° C.) between therotary cylinder 28 and the fixed surface 29. In the graph, the blacksquares represent points in the relationship when the rotary cylinder 28is made of silicon carbide and the fixed surface 29 is made of alumina,as in the first embodiment. The black diamonds represent points in therelationship when the rotary cylinder 28 and the fixed surface 29 areboth made of alumina. The relationship indicated by the black diamondsis referred to as a comparative example. The maximum temperaturedifference between the rotary cylinder 28 and the fixed surface 29 inthe operation range is represented by ΔTmax. The initial clearancedistance C is, for example, 3 to 6 μm. If the rotary shaft 13 is rotatedat 50,000 to 70,000 rpm, ΔTmax is about 80 to 120° C.

It is apparent from the graph of FIG. 4 that the decrease in theclearance distance C, which is caused by changes in the temperature, issmaller when the coefficient of thermal expansion of the material of therotary cylinder 28 is smaller than that of the fixed surface 29.

The coefficient of thermal expansion of the rotary cylinder and thefixed surface in the comparative example is 7 to 8×10⁻⁶/° C. Thecoefficient of thermal expansion of the rotary cylinder is thusrelatively large, and the difference between the coefficient of thermalexpansion of the rotary cylinder and that of the fixed surface is null.Therefore, in the comparative example, the clearance is eliminatedbefore the temperature difference ΔT reaches the maximum temperaturedifference ΔTmax.

In comparison, in the first embodiment, the coefficient of thermalexpansion of the material of the rotary cylinder 28 is 3 to 4×10⁻⁶/° C.,whereas the coefficient of thermal expansion of the material of thefixed surface 29 is 7 to 8×10⁻⁶/° C. The coefficient of thermalexpansion of the rotary cylinder 28 is relatively small, and thedifference between the two coefficients of thermal expansion is about4×10⁻⁶/° C. Thus, the reduction of the clearance distance C caused bychanges in the temperature is smaller than that of the comparativeexample. As a result, the clearance is not eliminated when thetemperature difference ΔT reaches the maximum temperature differenceΔTmax. Accordingly, contact between the rotary cylinder 28 and the fixedsurface 29 is prevented regardless of continuous high speeds, and afurther increase in the rotating speed is possible even if the initialvalue of the clearance distance C is 3-6 μm.

The desired effect is achieved as long as a ceramic having a lowcoefficient of thermal expansion of 5×10⁻⁶/° C. or less is used as thematerial of the rotary cylinder. The desired effect is guaranteed totake place when the coefficient of thermal expansion is 4×10⁻⁶/° C. orless. As long as the difference in the coefficients of thermal expansionbetween the rotary cylinder 28 and the fixed surface 29 is 1×10⁻⁶/° C.or more, the desired effect is obtained. The desired effect isguaranteed to take place when the difference between the coefficients ofthermal expansion is 2×10⁻⁶/° C. or more.

For example, since the coefficient of thermal expansion of zirconia is 7to 8×10⁻⁶/° C. and close to that of alumina, zirconia may be used as thematerial of the fixed surface 29.

The coefficient of thermal expansion of silicon nitride is 3 to 4×10⁻⁶/°C. and close to that of silicon carbide. Thus, silicon nitride may beused as the material of the rotary cylinder 28.

By using silicon carbide or silicon nitride as the material of therotary cylinder 28, heat dissipation of the rotary cylinder 28 isimproved since the coefficient of thermal conductivity of the rotarycylinder 28 is relatively high.

The first embodiment has the advantages described below.

The rotary cylinder 28 is formed from a ceramic material having acoefficient of thermal expansion that is smaller than that of the fixedsurface 29. Thus, the clearance may be set at a relatively small valueof several micrometers. Accordingly, the motor 10 and theturbo-molecular pump 1 can be made smaller and can be operated at higherspeeds.

Since the clearance is small, the turbo-molecular pump 1 achieves ahigher degree of vacuum at high rotating speeds.

An oxide ceramic such as alumina or zirconia is used as the material ofthe fixed surface 29. Thus, the machining and manufacturing of the airbearing 30 is relatively simple, which reduces cost.

The rotary cylinder 28 is made of silicon carbide or silicon nitride.Thus, the coefficient of thermal expansion of the rotary cylinder 28 issmall and heat is easily dissipated. This reduces the temperature of therotary cylinder 28 and limits the reduction of the clearance distance C.Accordingly, the motor 10 can be operated at higher rotating speeds.

The O-rings 11, 12, which are made of rubber, elastically connect themotor 10 to the housing 2. This absorbs the vibrations of the motor 10that would otherwise be transmitted to the housing 2.

A turbo-molecular pump according to the present invention will now bedescribed. In the second embodiment, the material of the rotary cylinder28 differs from that of the fixed surface 29. The rotary cylinder 28 ismade of silicon carbide, and the fixed surface 29 is made of siliconnitride. The coefficient of thermal expansion of the rotary cylinder 28is 3 to 4×10⁻⁶/° C., which is about the same as that of the fixedsurface 29.

FIG. 5 is a graph showing the relationship between the clearancedistance C (μm) and the temperature difference ΔT (° C.) between therotary cylinder 28 and the fixed surface 29. The black circles representpoints in the relationship when the rotary cylinder 28 is made ofsilicon carbide and the fixed surface 29 is made of silicon nitride. Inother words, the circles represent the second embodiment. As shown inFIG. 5, the reduction in the clearance distance C that is caused bychanges in the temperature is smaller than that of the comparativeexample. In the second embodiment, the difference between thecoefficients of thermal expansion of the materials used for the rotarycylinder 28 and the fixed surface 29 is substantially null. However, thecoefficient of thermal expansion of the rotary cylinder 28 is 3 to4×10⁻⁶/° C. and relatively small. Thus, the clearance is not eliminatedwhen the temperature difference ΔT reaches the maximum temperaturedifference ΔTmax. Accordingly, contact between the rotary cylinder 28and the fixed surface 29 is prevented at continuous high speeds and afurther increase in the rotating speed is possible.

In the second embodiment, the material combination of the rotarycylinder 28 and the fixed surface 29 may be changed to silicon carbideand silicon carbide, silicon nitride and silicon nitride, or siliconnitride and silicon carbide, respectively. These material combinationsalso achieve the necessary clearance and permit high speed rotationwithout contact between the rotary cylinder 28 and the fixed surface 29.

When the coefficients of thermal expansion of the rotary cylinder 28 andthe fixed surface 29 are substantially the same, the desired effect isachieved if the material of the rotary cylinder 28 has a coefficient ofthermal expansion of 5×10⁻⁶/° C. or lower. The desired effect is furtherguaranteed if the material has a coefficient of thermal expansion of4×10⁻⁶/° C. or lower.

The materials used for the rotary cylinder 28 and the fixed surface 29are carbides or nitrides, which have coefficients of thermalconductivity that are higher than those of oxides. The rotary cylinder28 and the fixed surface 29 thus have improved heat dissipation andavoid contact.

A turbo-molecular pump according to a third embodiment of the presentinvention will now be described with reference to FIGS. 6A to 8.

As shown in FIG. 6B, like the first embodiment, there are V-shapedgrooves 45 a, 45 b, a helical groove 46, and annular grooves 47 a, 47 bin the rotary cylinder 28. The depth of each type of groove differs fromthat of the other types. FIG. 7 is a graph showing the relationshipbetween the depth of the helical groove 46 and the pressure at the inletof the groove (Pa). The groove inlet pressure is the pressure that actson the upper end (right side as viewed in FIG. 6A) of the helical groove46. It is preferred that the value of the groove inlet pressure be assmall as possible. More specifically, it is preferred that the grooveinlet pressure be 10² Pa or lower. The lower end of the helical groove46 is exposed to the compressed air layer. Thus, it is required that thehelical groove 46 (gas seal area 44) function as an air seal under apressure difference of 10² Pa or more.

As apparent from FIG. 7, the seal effect of the gas seal area 44 dependson the depth of the helical groove 46. The desired seal is obtained whenthe depth of the helical groove 46 is two to ten times the clearancedistance C. To obtain a high sealing effect, the preferred depth of thehelical groove 46 is four to eight times the clearance distance C.

The relationship between the depth of the V-shaped grooves 45 a, 45 band the natural frequency of the rotary element 25 will now bedescribed. The motor 10 is used within a range of about 60,000 to 90,000rpm. Accordingly, the rotating speed, or frequency, of the rotaryelement 25 during operation is 1,000 to 1,500 rps (Hz). To avoidresonance, the value of the natural frequency of the rotary element 25(in Hz) must differ greatly from the operation speed (in RPS).

As apparent from FIG. 8, the natural frequency of the rotary element 25decreases as the depth of the V-shaped grooves 45 a, 45 b increases.When the depth of the V-shaped grooves 45 a, 45 b is one to five timesthe clearance distance C, the natural frequency is not included in theoperation speed range. If the depth of the V-shaped grooves 45 a, 45 bis 6C, the natural frequency of the rotary element 25 approaches theoperation speed range (about 1000 to 1500 Hz). In such state, resonanceis apt to occur and smooth rotation of the rotary shaft 13 may behindered. Accordingly, the preferred depth of the V-shaped grooves 45 a,45 b is one to five times the clearance distance C.

To smoothly draw air through the apertures 48 and into the clearance,the depth of the annular grooves 47 a, 47 b must not exceed a value thatis two times the depth of the V-shaped grooves 45.

The depth of the annular grooves 47 a, 47 b is required to exceed twotimes the depth of the V-shaped grooves 45 a, 45 b. Thus, to facilitatemanufacture, it is preferred that the clearance distance C be three tofifteen times the depth of the V-shaped grooves 45 a, 45 b. Themanufacture of the rotary cylinder 28 is especially facilitated when thedepth of the annular grooves 47 a, 47 b is equal to the sum of the depthof the V-shaped grooves 45 a, 45 b and the depth of the helical groove46.

When the depth of the V-shaped grooves 45 a, 45 b is represented byA_(PS), the depth of the annular grooves 47 a, 47 b by A_(R), and thedepth of the helical groove 46 by A_(GS), the range of each value isA_(PS)=1×C to 5×C, A_(GS)=2×C to 10×C, and A_(R)=3×C to 15×C. It isespecially effective if A_(GS) equals 4×C to 8×C. It is preferred thatthe depth combination satisfy the relationship of A_(R)=A_(PS)+A_(GS).The preferred clearance distance C is about ten micrometers or less.

The third embodiment has the advantages described below.

When the motor 10 is driven, the V-shaped grooves 45 a, 45 b function toform a high-pressure compressed gas layer with the air drawn in throughthe apertures 48 at the axially middle portion of each air bearing area43 a, 43 b. The depth of the annular grooves 47 is set at A_(R)=3C to15C. During rotation, air flows smoothly from the apertures 48 to theclearance. Thus, the compressed gas layer is readily formed. When therotating speed approaches 5,000 rpm, the compressed gas layers cause therotary element 25 to float away from the inner surface of the fixedsurface 29. That is, the rotary element 25 is supported by the airbearing 30.

The depth of the V-shaped grooves 45 a, 45 b is set at A_(PS)=1C to 5C.Thus, the natural frequency of the rotary element 25 differs greatlyfrom the operation speed range and prevents the rotary element 25 fromresonating. Accordingly, satisfactory bearing characteristics areobtained. Further, the rotary shaft 13 is stable in the operation speedrange.

The depth of the helical grooves 46 is set at A_(GS)=2C to 10C. Thus, ahigh vacuum degree of 10² Pa or lower is guaranteed. If the condition ofA_(GS)=4C to 8C is satisfied, a vacuum degree of 10 Pa or lower isobtained.

The outer helical groove 17 a increases the degree of vacuum (about 1 Paor less). In addition, the relative rotation between the rotor vanes 16and the stator vanes 7 functions to further decrease the pressure of theintake region thereby achieving an ultra-high vacuum. In other words,the degree of vacuum generated by the turbo-molecular pump 1 isdetermined not only by the depressurization capability of the rotorvanes 16 and the stator vanes 7 but also by the seal characteristics ofthe outer helical groove 17 a and the gas seal area 44. In the thirdembodiment, the seal characteristics of the helical groove 46 and theouter helical groove 17 a are improved. Thus, the turbo-molecular pump 1produces a greater vacuum.

The depth of the annular grooves 47 a, 47 b is set equal to the sum ofthe depth of the V-shaped grooves 45 a, 45 b and the depth of thehelical groove 46 (A_(R)=A_(PS)+A_(GS)). Thus, three types of grooves 45a-47 b having different depths may be formed through two groove formingprocesses.

It should be apparent to those skilled in the art that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. Particularly, it should beunderstood that the present invention may be embodied in the followingforms.

In the first embodiment, any kind of material may be used as long as thecoefficient of thermal expansion of the material of the rotary cylinder28 is smaller than that of the material of the fixed surface 29. Forexample, the rotary cylinder 28 may be made of alumina and the fixedsurface 29 may be made of zirconia. Further, boron nitride or aluminumnitride may be used as the material of the rotary cylinder 28 or thefixed surface 29.

In the first embodiment, the ceramic material of the rotary cylinder 28and the ceramic material of the fixed surface 29 is not limited tomaterials in which the difference in the coefficients of thermalexpansion is equal to or greater than a predetermined value. As long asthe coefficient of thermal expansion of the material of the rotarycylinder 28 is 5×10⁻⁶/° C. or lower, a material combination having asmall difference in the coefficients of thermal expansion also maintainsthe clearance of the air bearing 30 in the operation speed range.

In the third embodiment, due to the anti-wear characteristics and heatcharacteristics, ceramic is preferred as the material for the rotarycylinder 28 and the fixed surface 29. However, at least one of therotary cylinder 28 and the fixed surface 29 may be made of plastic aslong as the material has the necessary anti-wear and heatcharacteristics. Further, the material of the rotary cylinder 28 and thematerial of the fixed surface 29 may either be the same or different.

In the first and second embodiments, a ceramic oxide, such as mullite orzircon, which resist wear, may be used as the material of the fixedsurface 29.

In the first and second embodiments, cordierite, which is an oxidehaving a low coefficient of thermal expansion, may be used as thematerial of the rotary cylinder 28.

In the first and second embodiments, a static air bearing may beemployed as the air bearing 30.

In the first and second embodiments, an air bearing may be employed inlieu of the magnetic bearings 37, 38, which support the rotary shaft.

In the first and second embodiments, the outer helical groove 17 a maybe eliminated.

In the third embodiment, the depth of the annular grooves 47 a, 47 b isnot limited to the sum of the depth of the V-shaped grooves 45 a, 45 band the depth of the helical groove 46. As long as the annular grooves47 a, 47 b have a depth that is two times the depth of the V-shapedgrooves 45 a, 45 b, a sufficient amount of air is provided to theclearance and a high-pressure compressed gas layer is formed.

In the third embodiment, the depth of the annular grooves 47 a, 47 b maybe changed as required. However, if the depths of the V-shaped grooves45 a, 45 b and the helical groove 46 is set in the same manner as thethird embodiment, the seal and bearing characteristics are improved.

In the first to third embodiments, at least one of the dynamic pressuregrooves 45 a, 45 b and the helical groove 46 may be formed on the innersurface of the fixed surface 29.

In the first to third embodiments, the motor 10 may be applied to anapparatus other than the turbo-molecular pump 1, for example, acompressor.

In the first to third embodiments, the helical grooves 17 a, 46 extendin a direction corresponding to the rotating direction of the motor 10.However, if, for example, the motor 10 is employed in a compressor, thehelical grooves 17 a, 46 may extend in the opposite direction.

The present examples and embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalence of the appended claims.

What is claimed is:
 1. A motor comprising: a rotary shaft; and a bearingfor radially supporting the rotary shaft, wherein the bearing includes:a cylindrical rotary member connected to the rotary shaft; and acylindrical fixed surface surrounding the rotary member, wherein thefixed surface is spaced from the rotary member by a predetermineddistance, wherein at least one of the rotary member and the fixedsurface has a dynamic pressure groove formed on a predetermined firstarea defined on a surface opposing the other of the rotary member andthe fixed surface, and wherein at least one of the rotary member and thefixed surface has a seal groove formed on predetermined second areadefined on a surface opposing the other one of the rotary member and thefixed surface, the seal groove being formed deeper than the dynamicpressure groove; wherein the depth of the seal groove is within a rangeof two to ten times the predetermined distance.
 2. The motor accordingto claim 1, wherein the depth of the dynamic pressure groove is within arange of one to five times the predetermined distance.
 3. The motoraccording to claim 1, wherein seal groove is helical.
 4. The motoraccording to claim 1, wherein the motor is provided in a turbo molecularpump.
 5. The motor according to claim 1, wherein the motor is providedin a compressor.
 6. A motor comprising: a rotary shaft; and a bearingfor radially supporting the rotary shaft, wherein the bearing includes:a cylindrical rotary member connected to the rotary shaft; and acylindrical fixed surface surrounding the rotary member, wherein thefixed surface is spaced from the rotary member by a predetermineddistance; wherein at least one of the rotary member and the fixedsurface has a dynamic pressure groove formed on a predetermined firstarea defined on a surface opposing the other of the rotary member andthe fixed surface; and wherein at least one of the rotary member and thefixed surface has a seal groove formed on a predetermined second areadefined on a surface opposing the other one of the rotary member and thefixed surface; the seal groove being formed deeper than the dynamicpressure groove; wherein at least one of the rotary member and the fixedsurface has an annular groove formed between the first predeterminedarea and the second predetermined area, wherein the annular groove isdeeper than the seal groove.
 7. The motor according to claim 6, whereinthe depth of the annular groove is within a range of three to fifteentimes the predetermined distance.
 8. The motor according to claim 6,wherein the depth of the annular groove is substantially equal to thedepth of the seal groove and the depth of the dynamic pressure groove.9. The motor according to claim 6, wherein the motor is provided in aturbo molecular pump.
 10. The motor according to claim 6, wherein themotor is provided in a compressor.
 11. A turbo-molecular pumpcomprising: a housing; a stator vane attached to the housing; a rotorvane rotatable relative to the stator vane; and a motor for driving therotor vane, wherein the motor includes: a rotary shaft; and a bearingfor radially supporting the rotary shaft, wherein the bearing includes:a cylindrical rotary member connected to the rotary shaft; and acylindrical fixed surface surrounding the rotary member, wherein thefixed surface is spaced from the rotary member by a predetermineddistance, wherein at least one of the rotary member and the fixedsurface has a dynamic pressure groove defined on a surface opposing theother of the rotary member and the fixed surface, and wherein at leastone of the rotary member and the fixed surface has a first seal grooveformed on a surface opposing the other of the rotary member and thefixed surface, the first seal groove being formed deeper than thedynamic pressure groove, wherein the motor includes a generallycylindrical case, wherein the pump further comprises a cup-like wheelcoupled to a distal end of the rotary shaft to cover the case andsupport the rotor vane, the wheel having an inner cylindrical surfacethat is separated from a outer cylindrical surface of the case duringoperation of the motor, and wherein at least one of the wheel and thecase has a second seal groove formed on a surface opposing the other ofthe wheel and the case.
 12. The pump according to claim 11, wherein thesecond seal groove is helical.
 13. The pump according to claim 11,wherein the motor is elastically supported by the housing via an elasticmember.
 14. A motor comprising: a rotary shaft; and an oil-free bearingfor radially supporting the rotary shaft, wherein the bearing includes:a cylindrical rotary member connected to the rotary shaft; and acylindrical fixed surface surrounding the rotary member, wherein thefixed surface is spaced from the rotary member by a predetermineddistance, wherein at least one of the rotary member and the fixedsurface has a dynamic pressure groove formed on a predetermined firstarea defined on a surface opposing the other of the rotary member andthe fixed surface, and wherein at least one of the rotary member and thefixed surface has a seal groove formed on a predetermined second areadefined on a surface opposing the other one of the rotary member and thefixed surface, the seal groove being formed deeper than the dynamicpressure groove.
 15. The motor according to claim 14, wherein thebearing is an air bearing.
 16. The motor according to claim 14, whereinthe motor is provided in a turbo molecular pump.
 17. The motor accordingto claim 14, wherein the motor is provided in a compressor.
 18. A motorcomprising: a rotary shaft; and a bearing for radially supporting therotary shaft, where in the bearing includes: a cylindrical rotary memberconnected to the rotary shaft; and a cylindrical fixed surfacesurrounding the rotary member, wherein the fixed surface is spaced fromthe rotary member by a predetermined distance, wherein at least one ofthe rotary member and the fixed surface has a dynamic pressure grooveformed on a predetermined first area defined on a surface opposing theother of the rotary member and the fixed surface, the dynamic pressuregroove generating a layer of compressed gas on the first area when themotor is driven, and wherein at least one of the rotary member and thefixed surface has a seal groove formed on a predetermined second areadefined on a surface opposing the other one of the rotary member and thefixed surface to prevent leakage of the compressed gas from the firstarea, the seal groove being formed deeper than the dynamic pressuregroove.
 19. The motor according to claim 18, wherein the bearing isprovided in a turbo molecular pump.
 20. The motor according to claim 18,wherein the motor is provided in a turbo molecular pump.